Board.java

import java.util.*;

public final class Board {
    private static volatile Board instance;
    private final int size;
    private final Map<Integer, Entity> entityMap;

    private Board(int size, List<Entity> entities) {
        this.size = size;
        this.entityMap = new HashMap<>();
        int last = getLastCell();
        for (Entity e : entities) {
            // checks if the entity position is valid
            validate(e, last);
            if (entityMap.containsKey(e.getStart())) {
                throw new IllegalArgumentException("Duplicate entity at cell " + e.getStart());
            }
            entityMap.put(e.getStart(), e);
        }
    }

    private void validate(Entity e, int last) {
        if (e.getStart() <= 0 || e.getStart() > last || e.getEnd() <= 0 || e.getEnd() > last) {
            throw new IllegalArgumentException("Entity out of bounds");
        }
        if (e instanceof Snake && e.getEnd() >= e.getStart()) {
            throw new IllegalArgumentException("Snake must descend");
        }
        if (e instanceof Ladder && e.getEnd() <= e.getStart()) {
            throw new IllegalArgumentException("Ladder must ascend");
        }
    }

    public static Board getInstance(int size, List<Entity> entities) {
        if (instance == null) {
            synchronized (Board.class) {
                if (instance == null) {
                    instance = new Board(size, entities);
                }
            }
        }
        return instance;
    }

    public Optional<Entity> getEntityAt(int cell) {
        return Optional.ofNullable(entityMap.get(cell));
    }

    public int getLastCell() {
        return size * size;
    }

    public int getSize() {
        return size;
    }
}